Manufacture of magnetic materials



Patented May 14, 1940 UNITED STATES PATENT OFFICE I Olden,

Downers Grove,

11]., assignors to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application April 18, 1839, Serial No. 267,606

.4Claims.

This invention relates to the manufacture of magnetic materials, and more particularly to methods of producing finely divided magnetic materials for use in electrical signalling apparatus.

An object of this invention is the provision of improved methods for the production of finely divided magnetic materials possessing to a high degree those magnetic, electrical and physical properties which are desirable in magnetic cores employed in electrical signalling apparatus.

The magnetic materials produced in accordance with the invention are particularly adapted to the manufacture of magnetic cores of the insulated dust type in which particles of metal or alloy are coated with insulation and compressed into a body. Magnetic particles for these cores have been produced by casting the magnetic material into an ingot, hot rolling the ingot, and

7 29 then pulverizing the rolled material. This general procedure has been adapted to various metals and alloy compositions, but in some cases it has been difficult to produce sufiiciently small particles by these prior methods. For example, it is desirable for certain uses to provide magnetic alloys of the permalloy type in particles smaller than 5 microns in size and this invention is especially adapted to the manufacture of that type of product.

39 In one embodiment of the invention, a magnetic'alloy of the permalloy type is produced in uniform particles under 5. microns in size by melting the alloy ingredients with oxides and casting the melt into an ingot, hot rolling the ingot, grinding the rolled material into particles, thoroughly oxidizing the ground particles, pulverizing the oxidized particles into particles of the finally desired size, and finally reducing the oxide particles to the metallic state by heating in a reducing atmosphere.

The invention has .been successfully employed in the production of a molybdenum permalloy magnetic alloy comprising substantially 82% nickel, 16% iron and 2% molybdenum, and following is a detailed description of one specific embodiment of the invention as applied to that particular alloy composition.

In compounding this alloy, the nickel, iron and molybdenum in the proper proportions, and cal-- culated quantities of iron sulfide, iron oxide, or similar addition agents, are melted together. the melting operation is conducted in an open furnace, some oxidation of the alloy constituents will occur, depending in degree upon the heating sulfides of one or more of the constituent metals,

temperature and period, and the availability of atmospheric oxygen. After the molten material has been heated for a suflicient period, it is poured into an ingot mold and as soon as the material is solidified, the mold is removed and the 5 ingot is allowed to cool.

The ingot is next subjected to a hot rolling 'treatment to produce a fine crystalline structure therein. In this operation, the ingot is heated to about 240.0" E, passed continuously through successively reducing rolls until the material cools to a temperature below the recrystallization temperature of the alloy, and quenched in water.

After solidification and hot working of the alloy, a large proportion of the oxides, sulfides and 15 contained foreign materials are segregated at the boundaries of the grains and grain fragments. The quantities of sulfides and oxides that are added in the compounding operation are regulated so that an adequate but not excessive supply of intergranular material, including any oxides formed during melting, will be available in the alloy. The optimum quantities of these materials to be added will vary somewhat with the alloy composition and compounding procedure employed and in general the additions are controlled to minimize the presence of these materials within the j grain fragments of the rolled material. Because the final reduction in the size of the grain fragments is accomplished at a temperature below the recrystallization temperature of the alloy, the grains do not tend to grow in size after the final rolling and quench. Consequently, the rolled product has a stable crystalline structure in which the fine grain fragments consist of almost pure alloy demarked by the intergranular material.

The rolled material is crushed in a jaw crusher or hammer mill, and the crushed material is pulverized in an attritionmill or similar apparatus. In these operations, the alloy tends to fracture at the grain and grain fragment boundaries and this action is facilitated by the presence of the intergranular material. As iracture of the alloy progresses in the mill, the intergranular materials, which are present in very finely divided form, are exposed and released to the action of air currents set up by the milling action and the usual exhaust system on the mill.' The elimination of these materials is highly desirable be- ,cause foreign inclusions detract from the magnetic properties of the alloy. v

Due to the tendency of the alloy to fracture along its crystalline boundaries, it can be pulverized fairly rapidly into particles of the same order of magnitude as its grain fragments, but further reduction in particle size is difficult to accomplish. It appears that when the particles reach the stage in which they consist of relatively pure alloy, they tend to distort instead of fracture under the milling action. Experiments have shown that after the alloy has been worked in the mill for a period, the particle size reduction rate is greatly reduced and continued milling has a very slow effect on the particle size. Actually, it is objectionable to work the ductile particles excessively because the resultant distortion detracts from their magnetic properties unless followed by an anneal, and also complicates the subsequent application of insulation on the particles.

After the alloy has been milled for a suitable period, the resultant particles are sorted by sieving to select the material that has been sumcientiy comminuted. The proper sieve size varies with the exact compounding and rolling procedure employed, but a 200 mesh sieve is usually I satisfactory.

The oversize particles are re-milled or re-melt ed, and the selected particles, or those passing through the sieve, vare placed in a furnace and heated in the presence of oxygen until the particles are completely oxidized. The temperature and exposure period are interdependent and good results have been obtained by heating the particles for twelve hours at around 1200 F. in the oxygen. A somewhat higher temperature can be used and the period can be prolonged safely because thorough oxidation of the particles is desired and there is no ready danger of over-oxidation. Agitation of the particles throughout the operation promotes access of the oxygen to the individual particles and also prevents sintering, particularly when a relatively high oxidizing temperature is used.

After the particles are thoroughly oxidized, they are placed in a ball mill, or the like, and subjected to a pulverizing action in which the oxidized particles fracture readily into smaller particles. In this operation the reduction in particle size is substantially uniform and progressive so that the duration of the milling operation is determinable on the basis of the particle size finally desired. After a specified period of milling, the particles are comminuted to a predetermined size and each particle has substantially the same composition as the initially oxi dized particles, which in turn corresponds to the original alloy composition.

After the oxidized particles have been pulverized to the finally desired size, they are heated in the presence of hydrogen or other reducing agent until the particlesare reduced to the metallic state. The temperature'is maintained below the recrystallization temperature of the alloy at this stage. A temperature between 1000 F. and 1200 F. is satisfactory and in this range reduction of the particles in hydrogen is completed in about one hour. Agitation of the particles expedites the process and also deters sintering. Complete reduction of the particles to their metallic state is desirable because the presence of oxides detracts from the quality of the magnetic alloy in service.

X-ray examination of the reduced particles has shown that they are composed of an alloy corresponding in composition and proportions to the original alloy as compounded.

The particles at the conclusion of the reduc: tion process are somewhat porous and they are subjected to a compacting operation which involves working in a ball mill for a short period of one or two hours. This operation does not cause any appreciable fracturing of the material but it does compress the individual particles, which is beneficial, particularly in the subsequent insulating of the particles. The compacted particles may be coated with a suitable insulation, such as the mixture of magnesium silicate, sodium silicate and magnesium hydroxide disclosed in United States Patent 2,105,070, issued January 11, 1938, to A. F. Bandur, and then compressedat high pressure into the form of a core.

The resultant cores are particularly suitable for use with high frequency currents where eddy current losses are an important consideration. For certain applications of this type, the use of smaller particles in the core reduces the losses proportionally and for some purposes it is desir-- able to produce a core in which, the particles are smaller than 5 microns in size. With the described methods, particles of the desired size are produced readily and inexpensively. Due to the fracturing characteristics of the oxidized parti-. cles, the pulverizing process can be regulated to control the final particle size. The completed particles, despite their fineness, can be coated with insulation by established methods and with available materials. Contamination of the magnetic alloy is minimized and the particles exhibit excellent physical, electrical and magnetic properties which are reflected in improved operation of the apparatus in which the particles are incorporated. 1

Although the invention has been described as applied to a specific alloy composition, it will be apparent that modifications of the described methods adapted to a variety of metals and alloys are feasible, and it is to be understood that the invention is limited only to the scope of the appended claims.

What is claimed is:

l. A method of producing magnetic alloys in finely divided form for use in insulated dust magnetic cores comprising the steps of melting the alloy ingredients together to form an alloy, working said alloy to. cause it to fracture into particles, heating said particles in the presence of oxygen until they are thoroughly oxidized, pulverizing the oxidized particles into relatively small particles, and heating the small oxidized particles in the presence of a reducing agent to convert said particles to their metallic alloy.

2. A method of producing magnetic alloys of the nickel-iron type in finely divided form for use in insulated dust magnetic cores comprising the steps of comminuting the alloy into particles having a maximum size substantially greater than the finally desired size, heating said particles in the presence of oxygen until they are oxidized, pulverizing the oxidized particles into smaller particles, heating the small oxidizer particles in the presence of a reducing agent to re duce said particles to their metallic alloy state, and finally mechanically compacting the alloy particles to increase the capacity of the particles to receive and retain a subsequently applied coating of insulation.

3. A method of producing magnetic alloys in the form of particles under 5 microns in size suitable for use in insulated dust magnetic cores comprising the steps of comopunding an alloy of nickel, iron and molybdenum, grinding the alloy into particles having a maximum size substantially greater than 5 microns, heating said particles in the presence of oxygen until they are thoroughly oxidized, pulverizing the oxidized particles into particles smaller than 5 microns. then heating the oxidized particles in the presence of a reducing agent to convert them to their metallic alloy state, and finally working the particles sufliciently to compress without fracturing the particles to improve their capacity to receive and retain a subsequently applied coating of insulation.

4. A method of producing magnetic alloys of the nickel-iron type in the form of particles under 5 microns in size suitable for use in insulated dust magnetic cores comprising the steps of melting together approximately 82% nickel, 16% iron and 2% molybdenum, comminuting the resultant alloy into particles having a maximum size substantially greater than 5 microns, heating said particles at a temperature around 1200 F.

in the presence of oxygen until the particles are thoroughly oxidized, pulverizing the oxidized particles into particles under 5 microns in size, and finally heating the particles at around 1000* F. in

the presence of hydrogen to reduce said partim cles to metallic alloy form.

ROBERT P. CROSS. JR. ARTHUR N. OGDEN.

CERTIFICATE OF CORRECTION.

Patent No. 2,200,191. may 11;, 191p.

ROBERT P. cnoss, JR. ET AL.

It ishereby certified that error appears in the printed specification of theabov e numbered patent requiring correction as follows: Page 2, second column, lin 55, claiml, after the word "alloy" and before the period insert "stete"; line 62, claim- 2, for "oiidizer" read --oxidized--; and

thatthe said Letters Patent should be read with this correction therein I that-the same may conform to the record of the case in the Patent Office.

Signed and sealed this 1st day of October, A. D. 191m.

Henry Van Arsdale,

(S Acting Commissioner of Patents- CERTIFICATE OF CORRECTION.

Yatent No. 2,200,191, May 11;, 191p.

ROBERT P. cnoss, JR ET AL.

It is hereby certified that error apjaears in the printed specification {of the abov e numbered patent requiring correction as follows: Page 2, sec-- 0nd column, line 55; claim 1, after the word "alloy? and before the period.

ineert "Stete"; line 62, claim=2, for "oa zidizer" read --oxidized.-; and

that the said Letters Patent shouldbe read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this' let-day of OctoberQA. 1). 191m.

Henry Van Arsdale (Se l) Acting Commissioner of Patents. 

